Because of their superior magnetic properties, silicon sheet steels are widely used in the production of magnetic core components in electrical equipment such as motors, generators, transformers, and the like. These favorable magnetic properties, namely, high magnetic permeability, high electrical resistance and low hysteresis losses, will minimize wasteful conversion of electrical energy into heat, and will therefore permit manufacture of electrical equipment having greater power and efficiency. In order to effect and optimize the desired magnetic properties, however, the silicon sheet steels must be produced under carefully controlled and exacting processing parameters. Silicon sheet steels are therefore substantially more expensive than other more conventional flat rolled steel products.
In the high volume manufacture of small electrical equipment for consumer appliances, toys and the like, unit cost is perhaps the most important consideration, for outweighing equipment efficiency and power considerations. For these applications, therefore, electrical equipment manufacturers frequently utilize the less expensive, more conventional low-carbon sheet steels for magnetic core components. Hence, there is a considerable market for low-carbon sheet steels having acceptable magnetic properties for magnetic core applications.
In the course of producing low-carbon sheet steels for magnetic applications, economic considerations have dictated that expensive processing steps be avoided and that even inexpensive steps be minimized. Therefore, even though elaborate processes have been developed for producing low-carbon sheet steels having exceptional magnetic properties, such processes have not been adapted commercially, because the use of such processes would greatly add to the cost of the product, while not improving the magnetic properties of the resultant sheet sufficiently to equal those of silicon sheet steels having comparable cost of production. To be of any commercial value, therefore, any new process for improving the magnetic properties of low-carbon sheet steels must be one that will not significantly increase the steel's production cost. Commercially, therefore, low-carbon sheet steels for magnetic applications are produced from conventional low-carbon steel heats having less than 0.1 percent carbon and the usual residual elements at normal levels for cold-rolled products. The rolling procedures are similar to those used for other cold-rolled products. Specifically, the production steps are usually limited to hot-rolling a lowcarbon ingot to slab form; hot rolling the slab to sheet form; pickling the hot rolled sheet, cold rolling the pickled sheet for a reduction of 40 to 80 percent; and annealing the sheet to effect recrystallization, generally in a box annealing furnace. An optional final temper roll or stretch leveling of from 1/2 to 8 percent is sometimes provided for the purpose of improving core loss and/or flattening the resultant sheet to make it better suited for the end application, slitting and lamination stamping.
The commercially produced low-carbon sheet steels for magnetic applications of 18.5 mils thickness, and having a lamination anneal, typically exhibit permeabilities in the rolled direction of from 5000 to 6000 at 10 kilogauss, with core losses of from 1.3 to 1.6 watts/lb. For the same thickness at 15 kilogauss, permeabilities in the rolled direction typically range from 2000 to 4000 with core losses of 3.0 to 4.0 watts/lb. Sheets rolled to 25 mils typically exhibit permeabilities in the rolled direction of from 4200 to 4800, with core losses of 1.8 to 2.0 watts/lb. at 10 kilogauss; and permeabilities in the rolled direction of from 2000 to 3000 with core losses of 4.2 to 4.8 watts/lb. at 15 kilogauss.
These relatively wide ranges in magnetic properties reflect an established tendency on the part of industry to deemphasize magnetic properties in low-carbon sheet steel and emphasize low cost of production. Nevertheless, customers have recently begun to demand improved magnetic properties, particularly at 15 kilogauss, without an appreciable increase in cost. As noted above, producers have been hard pressed to improved magnetic properties in these steels without substantial increases in production costs.
One of the more costly steps in producing the low-carbon electrical sheet steels is the box annealing which is a rather protracted operation. In addition, box annealed coils usually have coil-set which necessitates subsequent leveling operations. Because of this, there have been efforts to utilize continuous annealing in place of the more conventional box annealing operation. This, of course, is much cheaper than the box anneal-temper roll and/or stretch level treatment and may eliminate the need for a leveling step. However, the continuous anneal treatment does not yield magnetic properties as good as the box anneal temper/stretch treatment, and hence few of these efforts have been utilized commercially without involving additional processing steps to improve magnetic properties.